Frank Solutions for Chapter 5 Heat Class 9 Physics ICSE
Exercise 5.1
1. What do you mean by heat?
Answer
Heat is defined as a form of energy which flows from one point to another on account of temperature difference.
2. Is heat a form of energy?
Answer
Yes, heat is a form of energy.
3. Name the SI unit of heat.
Answer
Joule is the SI unit of heat.
4. Define one joule of heat.
Answer
One calorie is defined as the quantity of heat required to raise the temperature of 1 gram of water through 1°C.
5. What is the relation between joule and calorie?
Answer
1 calorie = 4.2 joules.
6. Define temperature and write its SI unit.
Answer
Temperature is the degree of hotness or coldness of a body compared to other bodies around it.
SI unit of temperature is Kelvin (K).
7. Why does a piece of ice when touched with hands appear cold?
Answer
We feel cold on touching ice because heat flows from our warm hands to cold ice. Due to this flow of heat from hand to ice, the temperature of our hand falls. This is why we feel cold.
8. Distinguish between heat and temperature.
Answer
9. Two bodies at different temperatures are placed in contact with each other. State the direction in which heat flows.
Answer
Heat flows from a body at a higher temperature to a body of lower temperature.
10. Is it correct to say that heat is the cause of temperature?
Answer
Yes, heat is the cause of temperature because temperature of a body rises when the heat flows into the body.
11. What causes a change in the temperature of a body?
Answer
Heat changes the temperature of a body due to flow of heat in or out of the given body.
12. Which is more, a calorie or a joule?
Answer
Calorie. Because 1 calorie = 4.2 joules.
13. 1 joule = 4.2 calorie
Is this relation correct?
Answer
No, the exact relation is as given
1 calorie = 4.2 joules.
14. Two bodies are at same temperature. Do they necessarily contain same amount of heat?
Answer
Yes, because the heat flow is only due to temperature difference between the temperature of two bodies.
Exercise 5.2
1. What do you mean by temperature?
Answer
Temperature is the degree of hotness or coldness of a body compared to other bodies around it.
SI unit of temperature is Kelvin (K)
2. What is the normal temperature of human body?
Answer
Normal temperature of human body is 37°C.
3. Convert 20°C into Fahrenheit scale.
Answer
To convert 20° into°F
t°c /100 = (t°f - 32)/180
⇒ 20/100 = (t°f – 32)/180
⇒ 20 × 180/100 = t°f – 32
⇒ t°f = 36 + 32
⇒ t°f = 68°f
4. What is the upper fixed point on the Celsius scale?
Answer
Upper fixed point on the Celsius scale is 100°C.
5. Convert 80 K temperatures on the Celsius scale.
Answer
Tk = 80K
⇒ Tc = Tk - 273
⇒ Tc = 80 - 273
⇒ Tc = -153° C
6. Write down the SI unit of latent heat.
Answer
SI unit of latent heat is Joule per kg (J/kg).
7. What do you mean by relative humidity?
Answer
Relative humidity is defined as the amount of water vapour in the air compared to the amount needed for saturation.
8. Define coefficient of linear expansion. Write its SI unit.
Answer
Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C. Its SI unit is °c-1.
9. Name the scientist who designed the first thermometer.
Answer
Celsius was the scientist who discovered the first thermometer in 1710.
10. What is the principle of calorimetry?
Answer
According to principle of calorimetry of mixtures,
Heat gained = Heat lost
11. Write the SI unit of coefficient of cubical expansion.
Answer
SI unit of coefficient of cubical expansion is °c".
12. State two uses of a bimetallic strip.
Answer
Two uses of bimetallic strip are
- As thermostat in electric iron
- As balance wheel in watches
13. Why do telephone wires sag during summer?
Answer
Telephone wires sag in summer because due to heat of the sun, the wire expands and increases in length, thus they sag in summer.
14. State the types of thermal expansion. What is the relation between α and γ?
Answer
There are three types of thermal expansion
- Linear expansion
- Superficial expansion
- Cubical expansion
Coefficient of linear expansion be α
Coefficient of cubical expansion be γ
γ = 3α
15. Do all substances expand on heating? Give examples.
Answer
Not all substances expand on heating. Some examples of substances which do not expand on heating are plastics, polythene and rubber.
16. What is evaporation? Why does it cause cooling?
Answer
Evaporation is the phenomenon of a change of a liquid into vapour without raising the temperature. Evaporation needs energy for phase change from liquid to gases. As water evaporates off your skin, it absorbs energy(heat) from the body to make the phase change to gas thus cooling the body.
17. State three factors affecting evaporation.
Answer
Factors affecting evaporation are
- Humidity- more the humidity less is the evaporation
- Surface area- more the surface area more is the evaporation
- wind- more the wind more is the evaporation
- temperature- more the temperature more is the evaporation
18. What are land and sea breezes? Explain with the help of a labelled diagram.
Answer
The cold air that blows from land towards sea during night, is called land breeze
The cold air that blows from the sea towards the land during the day is known as the sea breeze. These breezes are the examples of natural convection current.
19. Is conduction possible in gases?
Answer
No, the conduction is not possible in gases. Gases are bad conductors.
20. Is conduction possible in vacuum?
Answer
No, conduction is not possible in vacuum.
21. What is the velocity of thermal radiations?
Answer
The velocity of thermal radiations is equal to the speed of light i.e,.3 x 108 m/s.
22. Why do we wear woolen clothes in winter?
Answer
We wear woolen clothes in winter because woolen clothes have tiny pores and air is trapped in these pores and being a bad conductor, the trapped air obstructs the flow of body heat to the surroundings.
23. Why is a newly made quit warmer than an old one?
Answer
A newly made quilt is warmer than an old one because the cotton in the old quilt gets compressed and very little air will remain trapped in it, hence heat insulation is quite poor.
24. In cold countries, the water pipes are covered with poor conductors. Why?
Answer
In cold countries, water pipes are covered with poor conductors because poor conductor prevents water from freezing and thus prevent these pipes from bursting.
25. Name three devices which are used to detect heat radiations.
Answer:
Three devices used to detect heat radiations are
- Blackened bulb thermometer
- Differential air thermo scope
- Thermopile
26. What is meant by thermal expansion?
Answer
The increase in size of a body on heating is called thermal expansion.
27. What do you mean by linear expansion?
Answer
Linear expansion is the increase in length of a solid on heating.
28. Define coefficient of linear expansion.
Answer
Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°c.
29. What is a bimetallic strip?
Answer
A bimetallic strip consists of two metal strips- one with high coefficient of expansion and the other with low coefficient of expansion.
30. What is the unit of coefficient of linear expansion?
Answer
SI unit of coefficient of linear expansion is °c’.
31. Name the substance which contracts when heated from 0°C to 4°C.
Answer
Water is the substance which contracts, when heated from 0°C to 4°C.
32. Define coefficient of volume expansion.
Answer
Coefficient of volume expansion is equal to the change in volume of a rod of volume 1m3 when its temperature rises by 1°C.
33. What is the unit of coefficient of volume expansion?
Answer
SI unit of coefficient of volume expansion is °C-1.
34. State two uses of bimetallic strip.
Answer
Two uses of bimetallic strip are
- As thermostat in electric iron
- As balance wheel in watches
35. How can a glass stopper jammed in the neck of a bottle be removed?
Answer
We should heat the neck of the bottle because due to heating the neck will expand and loosen the stopper stuck in the neck. In this way, we can easily remove the stopper from the bottle.
36. Why does a thick glass tumbler crack when very hot water is poured in it?
Answer
When hot water is poured into a thick glass tumbler, it generally cracks because on pouring hot water in the tumbler the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the tumbler cracks.
37. Why does a substance expand on heating?
Answer
A substance is made up of molecules arranged in a lattice. On heating, the molecules vibrate faster in the lattice and bump into each other harder. So the distance between the molecules increases thus expanding lattice. Thus, the substances expand on heating.
38. What are the different types of thermal expansions?
Answer
There are three types of thermal expansion
- Linear expansion
- Superficial expansion
- Cubical expansion
39. A small gap is left between two iron rails of the railway track. Why?
Answer
Gaps are left in the railway tracks because the tracks gets heated during the day and as a result they increase in length. If the gaps are not provided, the railway line would buckle outward and may cause derailment.
40. Why are bridges made of steel girders put on rollers?
Answer
The beams of the bridges expand maximum during the summer days and contract maximum during the winter nights. If the beams are fixed at both ends on the pillars, they may develop crack due to expansion and contraction. To avoid this, beams are made to rest on rollers on the pillars to provide space for expansion.
41. What is the relation between α and γ?
Answer
Let Coefficient of linear expansion be α
Coefficient of cubical expansion be y
The relation between them is
γ = 3 α
42. A copper wire 10 cm long is heated from 20°C to 30°C. Find the increase in length of the wire, if coefficient of linear expansion of copper is 1.7 × 10-5°C-1?
Answer
Original length, | = 10 cm = 0.1 m
Rise in temperature, t = 30 - 20 = 10°C
Coefficient of linear expansion, a = 1.7 ×10-5 °C-1
Increase in length = I ×t × a
= 0.1 × 10 × 1.7 × 10-5
= 1.7 × 10-5 m
43. One liter of mercury at 10°C is heated to 30°C. Find the increase in the volume of mercury. The coefficient of cubical expansion of mercury is 1.8 × 10-4 0c-1
Answer
Volume of mercury, V = 1 liter
Rise in temperature, t = 30 - 20 = 20°C
Coefficient of volume expansion, y = 1.8 ×10-4°C-1?
Increase in volume = V ×t ×y
= 1 ×20 ×1.8 ×10-4
= 3.6×10-3 liter
44. Why is a ventilator provided in a room?
Answer
A ventilator is provided in a room because it helps in removing the hot air from the room and allows the fresh and cold air to come in.
45. Is it possible to heat a liquid or gas from above? Explain your answer.
Answer
No, it is not possible to heat a liquid or gas from above because the transfer of heat through convection takes place vertically upwards in liquids and gases.
So if they are heated from above, the liquid or gas at the top will only be heated because most liquids and gases are themselves bad conductor of heat so they cannot conduct heat from top layer to the bottom layer.
46. Explain the following:
(i) Water is heated generally from below.
(ii) Land becomes warmer than water during the day.
Answer
(i) Water is heated generally from below because water itself is a bad conductor of heat and the transfer of heat through convection take place vertically upwards.
(ii) Land becomes warmer than water during the day because water has more specific heat capacity so it absorbs the heat and heats up slowly but on the other hand land has less specific heat and it heats up faster than water.
47. State three main characteristics of a thermometric substance.
Answer
Main characteristics of thermometric substance are
- The substance should have high coefficient of expansion so that it is sensitive to the smallest change in temperature
- The substance should have uniform expansion all over its entire volume
- The substance should have minimum specific heat so that it absorbs minimum heat from the body under measurement.
48. Name a substance which is an insulator of heat.
Answer
Wood is an insulator of heat.
49. (i) Why in cold countries windows have two glass panes with a thin layer of air between them?
(ii) What is the relation between joule and calorie?
(iii) Is it possible to boil water in a thin paper container? Explain with a reason.
Answer
(a) In cold countries, windows are provided with two glass panes because in between these two glass panes, a thin layer of air is present: air being a bad conductor obstructs the conduction of heat from the room to outside.
(b) 1 calorie = 4.2 joules
(c) Yes, it is possible to boil water in a thin paper cup because when heated the heat in the paper cup is transferred to the water through convection and paper cup doesn't get sufficient heat to get burnt
50. What is the principle of the thermometer?
Answer
Thermometer works on the principle that substances expand on heating and contract on cooling. So we use a thermometric substance which expands and contracts uniformly.
51. State the advantages and disadvantages of mercury and alcohol as thermometric liquids.
Answer
Advantages of mercury and alcohol as thermometric liquid:
- They both are good conductors of heat.
- They have high coefficient of expansion thus are sensitive to the smallest change in temperature
- Their freezing points are very low and boiling point is high in case of mercury
Disadvantages f mercury and alcohol as thermometric liquid:
- Alcohol is transparent and this makes hard to read the thermometer.
- It does not have uniform expansion.
- Mercury is less sensitive than alcohol as its coefficient of expansion is less than alcohol.
- Alcohol is a volatile liquid.
52. What is meant by the lower and upper fixed points of a thermometer?
Answer
Lower point of a thermometer is the temperature at which ice starts melting at normal atmospheric pressure i.e. 0°C
Upper point of a thermometer is the temperature at which water just starts boiling at normal atmospheric pressure i.e. 100°C.
53. Draw a neat and labelled diagram of a clinical thermometer.
Answer
54. State one use of each of the following types of thermometers: (i) Laboratory thermometer, (ii) Clinical thermometer, (iii) Six's maximum and minimum thermometer.
Answer
(i) Laboratory thermometer is used to measure and observe the temperature of various chemical reactions
(ii) Clinical thermometer is used to measure human body temperature
(iii) Six's maximum and minimum thermometer is used in meteorology and horticulture.
55. Distinguish between a clinical and a laboratory thermometer.
Answer
Difference between laboratory and clinical thermometer.
56. Which is the temperature that is common in the Celsius and the Fahrenheit scale?
Answer
The temperature that is common in both clinical and Fahrenheit scale is 40°C
Derivation is as follows
Let the temperature be x
C/100 = (F - 32)/180
⇒ x/100 = (x - 32)/180
⇒ x × 180/100 = x - 32
⇒ 9/5x = x - 32
⇒ -4/5x = 32
⇒ x = - 40
57. Convert the following temperatures into Fahrenheit scale:
60°C, 100°, -40°C, 85°C
Answer
(a) 60°C
60/100 = (F - 32) /180
⇒ F = 6×18 + 32
= 110° F
(b) 100 ° C
100/100 = (F – 32)/180
F = 180×1 + 32
= 212°F
(c) - 40° C
-40/100 = (F – 32)/180
F = - 4×18 + 32
= 40° F
(d) 85°c
85/100 = (F – 32)/180
F = 85× 18/10 + 32
= 185°F
58. Convert the following temperatures into Celsius scale.
Answer
(a) 104 °F
C = (F – 32)× 100/180
C = 72 x 100/180
= 40 °C
(b) 95°F
C = (F – 32)× 100/180
= 63 x 10/18
= 35° C
(c) 113° F
C = (F – 32)× 100/180
= 81 x 10/18
= 45°C
(d) 32°F
C = (F – 32)× 100/180
= 0 x 10/18
= 0°C
59. Draw a neat and labelled diagram of a Six's maximum and minimum thermometer. Explain briefly, the working of this thermometer. Give a use of this thermometer.
Answer
Diagram of six’s maximum and minimum thermometer
60. Explain the three modes of transfer of heat.
Answer
Three modes of heat transfer are
- Conduction involves the transfer of heat from the hot end to the cold end from particle to particle of the medium.
- Convection is the transfer of heat from one body to another by actual movement of the particles of the medium
- Radiation is the transfer of heat from one body to another without the need of an intervening material medium
61. With the help of a diagram, show that water is a bad conductor of heat.
Answer
62. Write a short note on Land Breeze and Sea Breeze.
Answer
The cold air that blows from land towards sea during night, is called land breeze
The cold air that blows from the sea towards the land during the day is known as the sea breeze. These breezes are the examples of natural convection current.
63. A wooden knob and a metal latch on a door are both at room temperatures. Explain, why the latch is colder to touch.
Answer
A wooden knob and a metal latch are both being at same temperature but it feels colder to touch the latch because metal is a good conductor and as soon as we touch it heat from our hand flows to the latch and we feel cold while on the other hand wood is a bad conductor of heat, heat of our hand does not flow into it therefore it does not feel cold.
64. Draw a neat and labeled diagram of a vacuum flask (thermos flask). Explain, how it minimizes loss of heat by preventing conduction, convection and radiation.
Answer
The flask consists of double walled glass container with vacuum between the walls A and B to prevent heat loss due to conduction and convection as vacuum is the excellent insulator .to prevent heat loss by radiation, the inner side of the wall A and outer side of wall B is silvered. It has a narrow mouth which is closed by a non-conducting rubber stopper.
65. A spiral cut from a thick chart paper is pivoted at O and is held slightly above a burning candle (Fig. 1). You will notice that the spiral starts moving. Explain.
The spiral starts moving because due to the flame of the candle the spiral heats up and expands. While expanding, the spiral tries to create space for the extension in length and an outward pull is created which causes the spiral to move.
66. Give scientific reasons for the following:
(i) In winter the human body covered with a blanket keeps warm.
(ii) It is better to use thin blankets to keep the body warm rather than using a single blanket of thickness equal to their combined thickness.
(iii) In winter the birds fluff up their feathers.
(iv) Old quits are less warm than the new ones.
(v) On a hot sunny day, it is advisable to wear light coloured clothes.
Answer
(i) In winters, the human body covered with a blanket keeps warm because the blanket has air trapped in it which provide heat insulation to the body from the surroundings and keep us warm.
(ii) It is better to use two thin blankets to keep the body warm rather than using a single blanket of equal thickness because in between the two thin blankets there is more air trapped than in the single blanket of equal thickness so using two thin blankets better heat insulation is provided to the body from the surroundings and keep us warm
(iii) In winter the birds fluff their feathers in order to trap air in their feathers so that the air provides heat insulation to their body from the surroundings and keep them warm and save them from winter.
(iv) Old quilts are less warmer than new ones because the cotton in the old quilt gets compressed and very little air will remain trapped in it, hence heat insulation is quite poor
(v) People wear light coloured clothes in winter because these clothes reflect most of the sun's radiations and absorb only a little of them. Therefore, they keep themselves cool.
67. What is meant by energy flow?
Answer
Transformation of Sun's energy in sun-eco system through a food chain is called energy flow.
68. Draw a flow chart to establish that the transfer of sun's energy in a sun-eco system combination in not cyclic.
Answer
69. State reason why any energy transfer cannot be 100%.
Answer
Any energy transfer is not 100% because energy is lost to the surroundings in the form of heat, friction losses during the transfer of energy. Therefore complete energy is not transferred.
70. State briefly the functioning of a biogas power-source.
Answer
Bio gas is produced by the action of bacteria on decaying organic matter. The primary source of bio gas in villages is dung of cow, or buffalo. The bio gas is mostly methane which can be used as chief source of light and heat energy.
71. The diagram (Fig. 2) shows two air filled bulbs connected by a U-tube containing mercury kept equidistant from a glassing bulb.
(i) Explain, how will the level X and Y change?
(i) Give reason for your answer.
72. The following diagrams illustrate three situations involving thermometers which are labeled A, B and C (Fig. 3). In each situation the thermometers indicate different readings.
(i) What do you expect the approximate reading of the thermometer B and C would be? Give a reason for your answer.
(ii) How would the readings of A and B help you in calibrating a thermometer?
(ii) Due to addition of salt boiling point of water increases so thermometer has to measure temperature for a wider range and thus it helps in calibration of thermometer.
73. A hot metal ball is suspended from a string in a metal box as shown in fig. 4.
(i) What are the ways in which the ball loses heat?
(ii) Which side of the box will be the hottest and which the least hot after sometime? Explain.
(ii) Speed of heat radiation is equal to the speed of light and intensity of heat radiation does net depend upon direction so all side of the box would be equally het as the box is cubical.
74. Fig. 5 shows a metal cylinder, containing boiling water. One half side A, is polished and another half, B is painted black. Two thin metal sheets X and Y are painted black and have one rubber stopper fixed with wax on each sheet. These sheets are equidistant from the boiling water (container A, B) as shown in the diagram. What would you expect to happen after a few minutes? Give reason for your answer.
75. Distinguish between Mercury and Alcohol as a thermometric liquid.
Answer
Difference between mercury and alcohol as thermometric substance
76. Water is not used as a thermometric liquid. Why?
Answer
Water is not used as a thermometric liquid because It has low coefficient of expansion so it is less sensitive to temperature changes. Moreover, It is transparent thus making it difficult to read the thermometer and water evaporates with time thus producing error and also the freezing and boiling points are also low.
77. How can you increase the sensitivity of a thermometer?
Answer
The sensitivity of a thermometer can be increased by using a substance having high coefficient of expansion and uniform expansion so that its expands with the slightest change in temperature.
78. Explain the following:
(i) Why a thick glass tumbler is likely to crack when hot water is poured in it?
(ii) Why does not a Pyrex tumbler crack when hot water is poured in it?
Answer
(i) When hot water is poured into a thick glass tumbler, it generally cracks because on pouring hot water in the tumbler the inner surface heats up and expands more as compared to its outer surface. This unequal expansion between the two surfaces causes a strain and the tumbler cracks.
(ii) Pyrex glass tumbler does not crack on adding hot water because Pyrex glass has low coefficient of expansion. It does not expand less when hot water is added to the tumbler.
79. Water at 0°C is heated to 10°C. Sketch a temperature-volume graph to show the behavior on heating.
Answer
It can be seen from the graph that the volume of the water decrease from 0°C to 4°C and the volume is minimum at 4°C. After 4°C the volume increases with the increase in temperature.
80. Differentiate between clinical thermometer and laboratory thermometer.
Answer
Difference between laboratory and clinical thermometer
81. Study the following diagrams (Fig. 6) and write down your observations.
82. The temperature of two bodies A and B differ by 1°C. By how much will it differ on a Fahrenheit scale?
Answer
Temperature in °C = 1°C
C/100 = (F - 32)/180
F = 1× 18/10 + 32
= 33.8° F
83. On a faulty thermometer lower fixed point is marked at 10°C and upper fixed point is marked as 130°C. What will be the reading on this thermometer when it is placed in a liquid which is actually at 40°C?
Answer
Lower fixed point = 10°C
Upper fixed point =130°C
Range of thermometer = 130°C - 10°C
= 120°C
No of divisions = 100
So least count = 120/100 = 1.2°C
On actual thermometer 40°C would have 40 divisions
So, on this thermometer it would show
= 40× LC = 48°C
84. What is greenhouse effect?
Answer
The green house is referred to a glass house. The heat enters the house but cannot escape out, because the glass reflects the heat back to the inside of the house. This makes glass house warmer than the outside environment. This phenomenon is called green house effect.
85. How global warming occurs?
Answer
Global warming occurs due to the presence of carbon di oxide, CFCs, methane in the atmosphere. Carbon dioxide acts as a transparent gas to incoming shortwave radiations which the earth re-radiates into space. It, therefore traps the outgoing radiations thus warming lower atmosphere of the earth thereby causing global warming.
86. What are the harmful effects of global warming?
Answer
Harmful effects of global warming are
- The atmospheric temperature of earth would increase thereby making it difficult for a living being to survive
- It would melt down the polar caps thus increasing the size of the ocean and leading to floods, tsunami, etc.
- The increase in temperature would affect climate and rainfall thus affecting flora and fauna.
- Human beings would be vulnerable to diseases as microbes would get warmth to grow.
87. What is the reason behind rising of temperature inside a greenhouse?
Answer
The temperature in a green house rises because heat enters the house through the glass but cannot escape out, because the glass reflects the heat back to the inside of the house. This makes glass house warmer than the outside environment.